FIG. 1 diagrammatically illustrates the configuration of a conventional guided acoustic travelling wave lens device--one that employs a relatively narrowly dimensioned traveling wave channel. The devicers comprises a laser 10, the optical beam output 11 of which is focussed by a cylindrical lens arrangement 12 and deflected by a mirror 13 onto an acousto-optic beam deflector 14, to which an RF input signal is applied. The acousto-optically modulated beam is then reimaged by a further spherical-cylindrical lens arrangement 15 onto a traveling lens cell 16, than contains a traveling wave lens transport medium 17 and a traveling wave lens launching transducer 18. The scanned beam is then imaged onto an image collection medium, such as a photographic film 19.
In a number of applications, the acousto-optic waveguide may be configured as a reduced height, guided acoustic travelling wave lens (ATWL), such as the liquid ATWL configuration diagrammatically illustrated at 30 in FIG. 2. In this type of acoustic wave transmission architecture, a first end 32 of the ATWL 30 has an acoustic wave input aperture 34, to which an acoustic wave-launching piezo-electric transducer is coupled, at an input end of a relatively narrow (e.g., fluid-containing) channel 36, having a cross-section of width w and height h, where w&gt;&gt;h.
For a non-limiting illustration of examples of documentation describing guided acoustic traveling wave lens devices, attention may be directed to an article entitled: "Optical Beam Deflection Using Acoustic-Traveling-Wave Technology," by R. H. Johnson et al, presented at the SPIE Symposium On Optical, Electro-Optical, Laser and Photographic Technology, August 1976, FIG. 6 of which corresponds to FIG. 1, above, an article entitled: "Guided acoustic traveling wave lens for high-speed optical scanners," by S. K. Yao et al, Applied optics, Vol. 18, pp 446-453, February 1979, and the U.S. Pat. No. 3,676,592 to Foster.
In the device described in the Yao et al article, the device is a solid ATWL guided structure, but not an index guided device. The length of scan is limited by dispersion, and the structural strength is limited by the narrow guide height. In the device described in the Foster patent, the ATWL uses a liquid as the acousto-optic medium and not index guided. In general liquids are difficult to impedance-match and the liquid containment packaging is typically complex.
The diameter of the laser beam spot incident upon a ATWL may be on the order of one-quarter of the acoustic wavelength, and may have a diameter on the order of twenty to thirty microns, and the focal length of the acoustic travelling wave lens may be several tens of a wavelength. In this configuration, an acoustic wavelength of one millimeter or greater is required, to achieve a resolution gain in excess of a factor of ten.
A disadvantage of an unguided ATWL is the fact that, depending upon the length of the scan of the incident light beam, the height of the piezoelectric transducer and the acoustic lens may be on the order of ten to fifteen millimeters. This height, which is considerably greater than that required by the limited diameter light spot incident upon the device, is necessary in order to control the acoustic diffraction, as the travelling wave lens scans out away from the transducer. Even using a relatively tall transducer, there are still significant diffraction effects as the acoustic beam passes through the near field or Fresnel diffraction zone.